It is well known that organic materials such as plastics, rubbers, lubricating oils, etc. are prone to oxidation and deterioration in the presence of oxygen. Oxidation of organic materials causes the loss of those intrinsic properties characteristic of the organic material. With a view to preventing deterioration, a variety of antioxidants and antiozonants have been developed; however, these stabilizers fail to prevent completely the deterioration of the desired properties of the materials to which they are added. Thus, those skilled in the art are constantly searching for new and more effective stabilizing systems which are useful for the protection of polymers and other organic materials.
Synergists have been known in the art for some time. See for example U.S. Pat. No. 3,492,336 which discloses use of a tetra-alkyl thioethyl thiodisuccinate with phenolic type antioxidants in the stabilization of polyolefins.
U.S. Pat. No. 4,254,020 discloses compounds such as 2,9-dihydroxy-4,7-dithia-5-methyldecamethylene bis[3-(dodecylthio)propionate]as synergists for phenolic antioxidants.
In U.S. Pat. No. 3,398,116 a thiocarboxylic acid thioether ester is used in combination with a limited group of phenolic antioxidants to stabilize poly-alphaolefins against oxidative degradation.
U.S. Pat. No. 3,758,549 discloses polyalkanol esters of alkylthio-alkanoic acids as synergists with phenolic antioxidants and U.S. Patent Nos. 3,666,716 and 3,505,225 disclose derivatives of diphenylamine and phenylnaphthylamines as antioxidants with dialkyl 3,3'-thiodipropionates as a synergist. U.S. Pat. No. 3,450,671 discloses polyolefin compositions stabilized with dialkyl 3,3'-thiodipropionate and a polyphenol.
An article by R. Chandra, Polymer, Vol. 24, February 1983 discloses styrene-butadiene copolymers stabilized with the antioxidant 3,5-di-t-butyl-4-hydroxybenzylmercaptan and 1,1,5-triphenyl-2-s-(3',5'-di-t-butyl-4'-hydroxy) benzyl-iso-4-thiobiuret as a polymer bound synergist.
U.S. Pat. No. 4,604,417 discloses polymerizable thioester synergists. The compounds of this reference possess a polymerizable moiety which allows for their incorporation into a polymeric network via copolymerization or grafting.
U.S. Pat. No. 3,661,822 discloses a stabilized EPDM polymer which contains 0.75 to 4 parts of a binary synergistic mixture of (A) and (B). (A) is an epoxide selected from the group consisting of: epoxidized soybean oil, epoxidized esters of fatty acids having 10 to 30 carbon atoms, epoxidized straight chain alpha-olefins, epoxidized polybutadiene and diglycidyl ether resin of 4,4'-isopropylidenediphenol; while (B) is an organic sulfide having the formula: R--A--R' wherein A is a radical selected from the group consisting of --S--, --S--S-- and --S--R"--S--; R and R' are alkyl, cycloalkyl, aryl, aralkyl, and alkaryl, and R" is selected from alkylene, arylene, alkylenearylene and heterocyclic divalent radicals. This patent fails to disclose or appreciate that the reaction between a material having epoxy or oxirane groups and a mercaptan will produce a compound that can synergistically enhance the activity of phenolic and amine antidegradants in the stabilization of organic compounds subject to oxidative degradation.
The prior art has suggested that for a compound to exhibit synergistic properties with an antioxidant or antiozonant, it would require a certain amount of mobility of the compound within and about the polvmer matrix. The present invention has unexpectedly discovered that high molecular weight synergists can be prepared by reacting a primary, secondary or tertiary mercaptan with a compound that contains at least one epoxy or oxirane group per molecule. Further, it has been discovered that the synergists of this invention demonstrate activity with what is now known in the art as bound antioxidants. This is highly unexpected and contrary to numerous and various teachings in the art.
The synergists of this invention can be produced through the facile reaction of an oil or polymer that possesses oxirane or epoxy groups with a mercaptan. The reaction product thus contains an organothioethyl alcohol moiety or segmer which can synergistically enhance the antidegradative properties of phenolic and amine antioxidants.
The majority of the synergistic stabilizers to date have been used in conjunction with phenolic antioxidants since use with amine antioxidants has not demonstrated synergistic properties. It was felt that synergism was simply overpowered by the excellent stabilizing properties of compounds such as N,N'-dialkyl-p-phenylenediamine. In this regard, it is surprising that the synergists of the instant invention demonstrate synergistic activity with amine type antioxidants. As a result of these discoveries, it has been found that the combined use of the synergists of this invention and an antioxidant brings about an unexpectedly powerful antioxidative effect. None of the cited patents or other literature in the art has disclosed or even suggests the use of the high molecular weight compounds disclosed in this invention as synergists in the stabilization of oxidizable organic materials.